Rapid development of mobile wireless communication and multimedia wireless communication in recent years requires large capacity and high speed data communication. A transmission amplifier for communication is required to have low distortion characteristics, which enable wide-band digital modulated waves to be amplified without distortion.
Distortions generated in an amplifier are roughly divided into harmonic components of a carrier frequency and components which appear in the vicinity of an amplified frequency. In general, components appearing in the vicinity of the carrier frequency cause a problem in communication. The harmonic components have very different frequencies, and hence those components can be removed by an external circuit such as a filter. However, in order to remove the components appearing in the vicinity of the carrier frequency, a very narrow band filter is required, which is difficult to realize in general.
Distortion appearing in the vicinity of the carrier frequency is generated when the high frequency signal to be amplified is modulated, and the modulation frequency causes a temporal change of the envelope. Further, this distortion is classified into two types including nonlinear distortion due to nonlinearity of the amplifier and memory effect distortion in which a past state of the amplifier is memorized and affects a current state like hysteresis characteristic.
The nonlinear distortion is caused by nonlinearity of the AM/AM characteristic or AM/PM characteristic of the amplifier at the carrier frequency. The memory effect distortion is generated in the case where distortion generated at other frequencies than the carrier frequency is cross-modulated with the carrier signal or the case where generation of distortion is different depending on a manner of a temporal change of the waveform. It is considered that the memory effect distortion is caused by an influence of heat, frequency characteristic of the amplifier, cross modulation with the harmonic component, cross modulation with distortion of a baseband frequency component generated in a bias circuit, or the like.
As one of methods for reducing the memory effect distortion, there is proposed a method of reducing the impedance of the bias circuit at a baseband frequency, so as to reduce distortion of the baseband frequency component and the cross modulation distortion with the carrier signal.
FIG. 11 is an explanatory diagram illustrating a mechanism in which the memory effect distortion is generated in the amplifier by distortion of the baseband frequency component. The amplifier for communication is usually set to a bias point of the class AB or C in order to realize low power consumption in the back-off region. Therefore, an instantaneous value of a drain current flowing in the transistor varies in accordance with an instantaneous input power of an input modulated signal, and the drain current varies at the baseband frequency of the input modulated signal.
A voltage variation at the drain terminal is expressed as a product of the drain current and the impedance at the baseband frequency of the bias circuit. Therefore, a drain terminal voltage VdFET varies at the baseband frequency in accordance with the impedance of the bias circuit. Thus, the carrier signal is modulated, and the memory effect distortion is generated. According to the above-mentioned mechanism, it is considered that the memory effect distortion can be suppressed by setting the impedance of the bias circuit at the baseband frequency to a value close to zero Ω.
FIG. 12 is a structure diagram of a conventional low distortion amplifier. Two short stubs having leading ends short-circuited with a high-frequency short-circuit element and a low-frequency short-circuit element (two-way bias network) is disposed in the vicinity of the drain terminal of the FET, so as to reduce the impedance of the bias circuit at the baseband frequency. As a result, the memory effect distortion is suppressed (see, for example, Non-patent Document 1).    Non-patent Document 1: Akio Wakejima, Kohji Matsunaga, Yasuhiro Okamoto, Kazuki Ota, Yuji Ando, Tatsuo Nakayama, and Hironobu Miyamoto, “370-W Output Power GaN-FET Amplifier with Low Distortion for W-CDMA Base Stations”, pp. 1360-1363, IEEE IMS2006